Will you be buying a new receiver for this room or using an existing receiver? This will shape some of the responses that you'll get here. If it's a new receiver situation, then I would say to go with 7.1. Right now, most movies don't have a 7.1 soundtrack. However, the receiver will still output 7.1 if set correctly. In my mind, there really is no good reason to go 5.1 if you are getting a new receiver and budget isn't severely limited. You don't have to have the rear surrounds immediately....could run it at 5.1 if you want to. The alternative is to buy 5.1 and get a case of upgradeitis in the near future when 6.1 and 7.1 tracks become more prevalent.

In the end, I am of the mindset that the more the speakers and enveloping sound the better. So, I'm going to fall into the 7.1 crowd.